use an immutable list for an lru_cached functions

When mutable items are stored in an lru cache, changing the returned
items changes the cached items as well. Therefore we want to ensure that
we're not mutating them. Using the ImmutableListProtocol allows mypy to
find mutations and reject them. This doesn't solve the problem of
mutable values inside the values, so you could have to do things like:

```python
ImmutableListProtocol[ImmutableListProtocol[str]]
```

or equally hacky. It can also be used for input types and acts a bit
like C's const:

```python
def foo(arg: ImmutableListProtocol[str]) -> T.List[str]:
    arg[1] = 'foo'  # works while running, but mypy errors
```

1 files changed, 3 insertions, 2 deletions

diff --git a/mesonbuild/cmake/interpreter.py b/mesonbuild/cmake/interpreter.py
index cc6adf1..f79f7d2 100644
--- a/mesonbuild/cmake/interpreter.py
+++ b/mesonbuild/cmake/interpreter.py
@@ -51,6 +51,7 @@ from ..mparser import (
 
 
 if T.TYPE_CHECKING:
+    from .._typing import ImmutableListProtocol
     from ..build import Build
     from ..backend.backends import Backend
     from ..environment import Environment
@@ -582,14 +583,14 @@ class ConverterTarget:
             self.compile_opts[lang] += [x for x in opts if x not in self.compile_opts[lang]]
 
     @lru_cache(maxsize=None)
-    def _all_source_suffixes(self) -> T.List[str]:
+    def _all_source_suffixes(self) -> 'ImmutableListProtocol[str]':
         suffixes = []  # type: T.List[str]
         for exts in lang_suffixes.values():
             suffixes += [x for x in exts]
         return suffixes
 
     @lru_cache(maxsize=None)
-    def _all_lang_stds(self, lang: str) -> T.List[str]:
+    def _all_lang_stds(self, lang: str) -> 'ImmutableListProtocol[str]':
         try:
             res = self.env.coredata.options[OptionKey('std', machine=MachineChoice.BUILD, lang=lang)].choices
         except KeyError: